1. Field of the Invention
The present invention relates to a gas component concentration measuring apparatus, and more particularly, to a gas component concentration measuring apparatus for detecting an oxygen concentration in gases to be measured, such as exhaust gases from an engine.
2. Description of Related Art
In an automotive engine, for example, the air/fuel ratio control is generally executed based on a detection result of an oxygen concentration (or an air/fuel ratio) by an oxygen concentration sensor. This oxygen concentration sensor has a solid electrolyte made of zirconia, and the temperature of the sensor element (or the solid electrolyte) has to be kept at a predetermined activation temperature so as to detect the oxygen concentration (or the air/fuel ratio) accurately with the solid electrolyte. Usually, a heater is attached to the sensor thereby to control its activation. A method for this heater control, as known per se, is to control the electric power fed to the heater or to feedback-control the temperature of the sensor element to a predetermined activation temperature.
In the related art described above, however, at a cold start of the engine, for example, the sensor element (or the solid electrolyte) is increased as soon as possible from the cold state. However, if the temperature of the sensor element is rapidly increased, certain disadvantages, such as an element or heater cracking or a separation of the bonded faces of the element and the heater, may result.
Furthermore, in recent years, there is a tendency that a regulation on the exhaust gas component of the engine is intensified more and more from the standpoint of environmental protection, and it has been desired to improve the detection accuracy and durability of the oxygen concentration sensor. According to the aforementioned related art, however, the individual differences or aging of the sensor compromise the sensor accuracy. This problem is caused by an excess or shortage of the heater calorific power necessary for maintaining the sensor activation state. In other words, if the heater has a poor calorific power, the desired sensor activation state cannot be maintained, and the sensor detection accuracy is compromised. On the other hand, an excessive calorific power deteriorates the sensor element or the heater.